1. Field of the Invention
The invention deals with a light guide that may be rectilinear or have a curved profile. The invention also deals with a lighting and/or signaling device, notably for a vehicle, the device comprising a light guide.
2. Description of the Related Art
In the field of motor vehicle lighting and signaling, it is increasingly commonplace to use optical guides. In practice, the optical guides present the advantage of being able to assume widely varied geometrical forms and bring a lighting surface even into inaccessible areas of the lighting and/or signaling device, thus contributing to the style of the device.
In the present application, the term “optical guide” is used to mean a transparent or translucent piece inside which light rays are propagated in a controlled manner from one of the ends of the guide, called “input face”, to at least one output face. The controlled propagation is generally performed by internal reflections on various faces, called internal reflection faces or guiding diopter.
In some light guides, the internal reflection faces may return the light toward an output face other than the terminal face, so that an observer has the impression that the output face is lit and that this output face corresponds to a light source. According to certain embodiments, for example as is described in the patent application DE 198 50 443 A1 and the patent application EP 1 780 463 A1, this output face is notably one of the internal reflection faces. In other words, the external surface of the guide forms, with the ambient air, a diopter suitable, on the one hand, for ensuring the propagation of the rays along the guide by total reflection and, on the other hand, for allowing a portion of these rays to leave for the lighting or signaling function. In this case, some incident rays on a first internal reflection face are returned with an angle of incidence to a second internal reflection face, the angle of incidence being such that, instead of being reflected on the second internal reflection face, these rays are transmitted and leave the guide. These first internal reflection faces can be obtained by the presence of a series of prisms on a rear face of the external surface of the guide, as is described in the documents DE 198 50 443 A1 and EP 1 780 463 A1. Such optical guides comprising prisms make it possible to obtain the emission of a light whose appearance is uniform on the output face, when the guide is in the on state. In the teaching of the document DE 198 50 443 A1, the section of light guide extending from the end receiving the light rays from the light source to the succession of prisms has a variable section intended to deflect the rays being reflected on the diopter formed by this section, in order to improve the standardization of the lighting produced by the guide.
A certain problem may however arise in a light guide conforming to the abovementioned teaching, as illustrated in FIGS. 1 to 4 of the present application. FIG. 1 illustrates a lighting or signaling device 2 comprising a light guide 4 extending in a generally curved longitudinal direction 5. A light source 6 is arranged facing a rear end of the light guide 4 forming an input face 8 for the light rays intended to pass through the light guide 4. The light guide 4 comprises a series of reflecting facets 16 on a rear face so as to decouple a portion of the rays passing through the light guide 4, which then leave through the front face 17 of the light guide 4 for a lighting or signaling function in a main direction 14 of the lighting or signaling device 2. The rays 20 and 22 emitted by the light source 6 symmetrically relative to the longitudinal direction 5 undergo at least one reflection of total reflection type on the diopter formed by the external surface of the light guide 4 to then be decoupled by the reflecting facets 16 and participate in the lighting beam. The surface of the light guide 4 in fact forms a diopter between the material of the light guide 4 with a given refractive index (typically of the order of 1.6 for polycarbonate) and the ambient air with a different refractive index (very close to 1). This refractive index difference between two contiguous media results in the existence of a limiting angle of incidence beyond which the refraction is impossible and for which there is total reflection. In the case of a medium of polycarbonate surrounded by air, this limiting angle is of the order of 39° (according to the Snell-Descartes law). In the case of polymethacrylate, the refractive index is of the order of 1.5 and the limiting angle approximately 42°.
The lighting or signaling device 2 also comprises a shield 12 arranged facing the front face 17 of the light guide 4, the rear portion of the light guide 4 comprising the input face 8 and the light source 6. The objective of this shield 12 is to collect spurious rays such as the ray 18 which are decoupled before encountering the active decoupling area with the reflecting facets 16. These rays 18 which are among the most inclined relative to the input face 8 meet the diopter with an angle of incidence less than the limiting angle mentioned above and leave the light guide 4 with refraction. They are likely to generate a non-uniform and non-esthetic lighting. It is therefore necessary to make them invisible. The image produced is illustrated in FIG. 2 which shows the device seen by an observer arranged in front of the device. The light guide 4 is lit uniformly and a series of vertical lines corresponding to the edges of the reflecting facets can be observed. The shield 12 well masks the rear portion of the light guide 4, its non-uniformities and the light source 6.
For technical reasons relating to the fastening of the light guide 4, the fastening of the light source 6 on the light guide 4 and/or the mold stripping of the light guide 4, it may be necessary to provide a portion of the light guide 4 with a variable cross-section, more particularly the end comprising the input face 8 in proximity to the light source 6. In practice, the fastening of the light guide 4 at its end on the side of its input face 8 and the fastening of the light source 6 at this end may impose a design notably comprising fastening pins and shoulders at the end of the light guide 4. Such shapes may require the light guide 4 to have a clearance suitable for allowing a mold stripping in the longitudinal direction of the light guide 4, that is to say, a clearance with oppositely inclined edges. A device comprising such a light guide 4 is illustrated in FIG. 3 in which the details of its end with the fastening means have been omitted for reasons of clarity of explanation. Numerous elements forming the devices illustrated in this figure correspond to those of FIG. 1. A consistent numbering has been adopted to designate these various elements bearing in mind that the reference signs of FIG. 3 correspond to those of FIG. 1, except that they are increased by 100. The same applies for FIG. 4. Specific numbers have been used to designate the elements that are not present in FIG. 1.
The device 102 comprises a light guide 104 with a generally curved profile 105 and comprising a section 124 with variable cross-section extending from the input face 108 to the succession of reflecting facets 116 intended for the progressive decoupling of the light rays. Like the device of FIG. 1, the device 102 also comprises a shield 112 arranged in front of the rear portion of the light guide 104 extending from the input face 108 to the reflecting facets 116, this portion corresponding to the portion with variable cross-section. The rays 120 and 122 emitted by the light source 106 symmetrically relative to the longitudinal direction undergo at least one reflection of total reflection type on the diopter formed by the section with variable cross-section 124 to then be decoupled by the reflecting facets and participate in the lighting beam. The incident ray 118 on the input face 108 with an angle a little greater than that of the rays 120 and 122 will undergo a first total reflection on a front area of the diopter and then a second total reflection on a rear area of the diopter to then again encounter the diopter in a front area with an angle of incidence less than or equal to the limiting angle. It will thus be refracted and leave the light guide 104 in a direction close to the main lighting direction 114 without encountering the shield 112. This unwanted decoupling is caused by the reduction of the cross-section of the section 124 of the light guide 104 which, through a set of successive reflections, progressively reduces the angle of incidence of the rays, relative to the normal to the diopter formed by the light guide 104. Based on various parameters such as the length of the section with variable cross-section and the variability of its cross-section along its longitudinal direction, a portion of the rays 120, 122 emitted by the light source 106 leaves the section with variable cross-section of the light guide 104 without encountering the shield 112, such as the incident ray 118. These rays 120, 122 will generate a area of higher light intensity 126 in proximity to the edge of the shield 112, as is illustrated in FIG. 4. Such an area renders the lighting beam strongly non-uniform and consequently detrimental from a photometric point of view and from an esthetic point of view.